Diesel engines are widely used for locomotive applications. These diesel engines typically include a turbocharger. As can be understood from FIG. 1, a conventional turbocharger 10 includes a compression stage 12 for compressing incoming air 16 and a turbine stage 14 for driving the compression stage using engine exhaust 30. The compression stage 12 takes the incoming air 16 and compresses it using a plurality of compressor blades 18 of a rotating compressor wheel 20 in conjunction with a stationary compression casing 22. The compressed air 36 is then expelled at a compressed air port 24. In order to rotatively drive the compressor wheel 20, the turbine stage 14 (see also FIG. 2) has a stationary turbine nozzle 26 composed of a plurality of turbine nozzle vanes 28 which direct the engine exhaust 30 onto a plurality of turbine blades 32 of a turbine wheel 34. The engine exhaust then vents through an exhaust air port 36. Since the turbine wheel 34 is drivingly connected to the compressor wheel 20, induced rotation of the turbine wheel provides rotation of the compressor wheel.
The diesel engine is typically operated at a set of throttle settings (or notches), each of which defines a specific engine load and speed for the locomotive. Each of these specific throttle settings cause the turbocharger to operate at discrete rotational speeds which correlate to the throttle settings. Also, the locomotive and engine control systems are typically designed to have safeguards which at times control the engine load and speed, turbocharger rotational speed, or other parameters which would also cause the rotational speed of the turbocharger to operate in a defined range.
It is a well-known physical property of metals that a fatigue failure will occur in a metal component if it endures a sufficiently large number of damaging stress cycles. A large number of stress cycles will occur over a short time period if the component is excited at one of its natural vibration (harmonic) frequencies. In this regard, the configuration (i.e., geometry) and material composition properties of the turbine blades define their natural frequencies.
One known source of vibrational excitation energy in turbochargers is aerodynamic excitation caused by movement of a turbine blade past the wake of a turbine nozzle vane. A turbine blade will pass a fixed number of the turbine nozzle vane wakes with each revolution of the turbine wheel. The number of turbine nozzle vanes and operating speed of the turbocharger will define the aerodynamic excitation frequency. The turbine blades are susceptible to high cycle fatigue failure if the excitation frequency or a harmonic of the excitation frequency is near one of the turbine blades natural vibration frequencies.
In that locomotive diesel engines must comply with current and future EPA emission regulations, there is a desire for the locomotive turbocharger to operate at high maximum speeds and new throttle settings of the engine. Problematically in this regard, a particular turbocharger may be operating in a diesel engine in which certain of the desired throttle settings may undesirably involve natural vibration frequencies of the turbine blades. This untoward situation would dictate that the locomotive and/or engine control system force operation of the engine only at throttle settings where the turbine nozzle induced excitation is not present. Unfortunately, this can result in the engine being operated below maximum power, at other than desired throttle settings and/or have sub-optimal fuel economy. Alternatively, one known “solution” is to utilize a turbine blade lacing wire which is installed through a respective hole in the turbine blades which dampens turbine blade vibration and thereby alters the natural vibration frequency of the turbine blades. Problematically, the manufacture and assembly associated with turbine blade lacing wire is significantly more expensive and complicated than a turbine stage fabricated without turbine blade lacing wire. Yet another known “solution” relates to using turbine blade contact with a turbine blade shroud. The turbine blade shroud is an attached (usually cast in) platform that is perpendicular to the axis of the turbine blades, and is in contact therewith. The turbine blade shroud contact with the turbine blades will alter the natural vibration frequency of the turbine blades, and friction caused by the mutual contact will damp vibrations. Unfortunately, this involves the same associated manufacturing and assembly issues as the lacing wire.
What remains needed in the art is a methodology for fabrication of the turbine stage of a locomotive turbocharger which ensures avoidance of natural vibration frequency of the turbine blades at desired throttle settings of the engine.